[HTML] pobierz, plaintext 
<script type='text/javascript'>
function check(obj)
{
     var proper_addr='http://www.youtube.com/';
     if (obj.value.substr(0,proper_addr.length)!=proper_addr) alert('Zły adres!');
}
</script>
<input onBlur=check(this)>
[HTML] pobierz, plaintext